1. Field of the Invention
The present invention relates to positive-working, radiation-sensitive mixtures which contain an organic, polymeric binder which is insoluble in water but soluble in aqueous alkaline solution and an IR-absorbing dye or pigment. It also relates to recording materials comprising a substrate and a layer of a mixture as described above, as well as to processes for the production of lithographic printing plates. Radiation-sensitive layers of the present invention have photosensitivity in the IR range so that recording materials prepared therewith are suitable, for example, for direct image production by the computer-to-plate (CTP) method.
2. Description of the Related Art
The use of dyes and pigments as IR absorbers in radiation-sensitive mixtures is generally known in the art. For example, the recording material according to WO 96/20429 comprises a layer containing IR-absorbing carbon black pigments, 1,2-naphthoquinone-2-diazidosulfonic esters or -carboxylic esters and a phenolic resin. 1,2-naphthoquinone-2-diazidosulfonic acid or -carboxylic acid can also be esterified directly with the hydroxyl groups of the phenol resin. The layer is first exposed uniformly to UV radiation and then imagewise to IR laser beams. As a result of the action of the IR radiation, specific parts of the layer rendered soluble by the UV radiation become insoluble again. This is therefore a negative-working system. The processing of the material is thus relatively complicated.
EP-A 0 784 233 also describes a negative-working mixture which contains a) novolak and/or polyvinylphenol, b) amino compounds for curing the component a), c) a cyanine and/or polymethine dye which absorbs in the near IR range and d) photochemical acid formers.
The non-prior-published patent application DE 197 39 302 describes a positive-working, IR-sensitive mixture which comprises a binder which is insoluble in water but soluble or at least swellable in aqueous alkali and carbon black particles dispersed therein, the carbon black particles being the radiation-sensitive component important for imagewise differentiation.
WO 97/39894 describes layers which contain dissolution-inhibiting additives. Such additives reduce the solubility of the unexposed parts of the layer on development in aqueous alkaline solutions. The additives are, in particular cationic compounds, especially dyes and cationic IR absorbers, such as quinolinecyanine dyes, benzothiazolecyanine dyes or merocyanines, in addition to various pigments. However, if these layers are heated to 50 to 100xc2x0 C. for from 5 to 20 seconds, the additives lose their dissolution inhibiting activity.
The positive-working mixture disclosed in EP-A 0 823 327 contains, as IR absorbers, cyanine, polymethine, squarylium, croconium, pyrylium or thiopyrylium dyes. Most of these dyes are cationic and have an inhibiting effect. Moreover, many of them are halogen-containing. Under unfavorable conditions, particularly upon IR irradiation or during baking, environmentally harmful decomposition products can form therefrom. Some dyes containing betaine groups and an anionic dye (compound S-9 on page 7) are also disclosed. After drying of the layer, however, due to its large number of sulfonated groups, the presence of this anionic dye generally causes crystallization or precipitation of components of the layer, leading to substantially poorer properties of the IR-sensitive layer and resulting in a poor appearance of the layer.
The disadvantage of the layer compositions known from the art is that the increase in solubility which is achieved by post-bake is reversible after storage at room temperature. If a printing plate is not further processed immediately after heating (for example, by using a heating oven), the development properties change, which may lead to reproduction problems in the processing of the recording materials. As already mentioned, environmentally harmful decomposition products may even form under unfavorable conditions as a result of halogen-containing cationic additives.
It was one object of the present invention to provide radiation-sensitive mixtures of the type described at the outset which contain neither diazonium compounds nor heat-curable or acid-curable amino compounds nor any silver halide compounds. Apart from an imagewise exposure and development, it is desirable that mixtures of the present invention generally require no additional operation, such as post-bake or postexposure, which was also an object of the present invention.
These and other objects can be achieved by a positive-working, radiation-sensitive mixture which contains an organic polymeric binder which is insoluble in water but soluble or at least swellable in aqueous alkaline solution and at least one IR-absorbing cyanine dye having a betaine structure or having a betaine structure and containing an anion, the cyanine dye having the formula (I) 
wherein
R1 to R8 independently of one another, comprise a hydrogen or halogen atom, a sulfonate, carboxylate, phosphonate, hydroxyl, (C1-C4)alkoxy, nitro, amino, (C1-C4)alkylamino, di-(C1-C4)alkylamino group or a (C6-C10)aryl group, which in turn may be substituted by one or more halogen atoms and/or one or more sulfonate, carboxylate, phosphonate, hydroxyl, (C1-C4)alkoxy, nitro, amino, (C1-C4)alkylamino and/or di(C1-C4)alkylamino groups,
R9 and R10 independently of one another, comprise a straight-chain or branched (C1-C6)alkyl, a (C7-C6)aralkyl or a (C6-C10)aryl group, each of which in turn may be substituted by one or more halogen atoms and/or one or more sulfonate, carboxylate, phosphonate, hydroxyl, (C1-C4)alkoxy, nitro, amino, (C1-C4)alkylamino and/or di(C1-C4)alkylamino groups,
R11 and R12 independently of one another, comprise (C1-C4)alkyl or (C6-C10)aryl groups, which in turn may be substituted,
Z1 and Z2 independently of one another, comprise a sulfur atom, a di(C3-C4)alkylmethylene group or an ethene-1,2-diyl group and
A comprises a carbon atom or a chain having conjugated double bonds which results in the formation of a delocalized xcfx80-electron system between the quaternary nitrogen atom of the 3H-indolium, quinolinium or benzothiazolium radical and the enolate oxygen atom of the pyrimidine-2,4,6-trione radical.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
A chain having the conjugated double bonds is in general 3 to 15 carbon atoms long. A delocalized relectron system in some embodiments usually also extends between the two bicyclic ring systems. Preferred dyes include those having a symmetrical structure, i.e. those in which the (partly) aromatic radicals in the formula (I) are substituted in the same way and in which n=m. They are also generally easier to synthesize.
The (C1-C4)alkoxy group is preferably a methoxy or ethoxy group, while the (C7-C16)aralkyl group is preferably a benzyl group. The halogen atoms are generally chlorine, bromine or iodine atoms. R11 and R12 are preferably methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, phenyl or naphth-1-yl or naphth-2-yl groups. The two radicals R11 and R12 are particularly preferably identical and are also particularly preferably being methyl groups.
The compounds of the formula (I) are referred to as xe2x80x9chaving a betaine structurexe2x80x9d because, in addition to the quaternary nitrogen atom of the 3H-indolium, quinolinium or benzothiazolium ring, they contain the pyrimidine-2,4,6-trione-enolate group shown in the formula. Carboxylate, sulfonate and/or phosphonate groups may also be present, so that the compounds as a whole may contain an anion and have a betaine structure. The number of these anionic groups should in general be not more than 5. The opposite ions of these anionic groups are generally alkali metal or alkaline earth metal cations, especially sodium or potassium ions, in addition to ammonium ions or mono-, di-, tri- or tetraalkylammonium ions. If amino, (C1-C4)alkylamino or di(C1-C4)alkylamino groups are present in the cyanine dye of the formula I, the number thereof is preferably less than or at most exactly the same as the number of carboxylate, sulfonate and/or phosphonate groups, so that the dye still has a betaine structure and contains an anion or still has a betaine structure.
Exemplary cyanine dyes having a betaine structure or having a betaine structure containing an anion include those having the following formulae (II) to (IV) 
wherein
n and m are integers from 1 to 8, with the proviso that n+m=2 or greater, and
Q are the members required for the formation of a 4- to 7-membered isocyclic or heterocyclic ring.
The dyes of formulae II-IV are preferable in some embodiments.
The ring formed under inclusion of Q in formula III and IV is preferably a (C4-C7)cycloalkene, particularly preferably cyclopentene. The 4- to 7-membered ring may also be substituted, in particular by halogen atoms, hydroxyl groups, alkoxy groups, nitro groups, amino groups, alkylamino groups, dialkylamino groups, carboxyl groups, sulfo groups or phosphonic acid groups. The heteroatoms include, in particular nitrogen, oxygen and/or sulfur atoms. A plurality of heteroatoms may also occur in the ring.
Finally, in addition to the compounds of the formulae (III) and (IV), structurally isomeric compounds in which the enolate of the pyrimidine-2,4,6-trione is not bonded to the 4- to 7-membered isocyclic or heterocyclic ring but to a carbon atom of the carbon chain linking the two bicyclic radicals are also suitable. Furthermore, dyes in which n=m=1 are particularly preferred in some embodiments.
In mixtures according to the invention, the IR-absorbing cyanine dyes F1 to F3 having a betaine structure or having a betaine structure and containing an anion as shown below (the cationic dye F4* is included for purposes of comparison and is therefore marked with *) are particularly suitable. 
Surprisingly, it has been found that the IR-absorbing additives having a betaine structure and containing an anion still have no solubility-inhibiting effect on the layer, but as a rule promote the dissolution or swelling rate when used in aqueous alkaline developers. IR-absorbing additives having a betaine structure can have an inhibiting effect but are relatively inert after a brief post-bake, i.e. they experience no increase in solubility in aqueous alkaline developers.
The amount of the IR-absorbing dye is advantageously from 0.2 to 30% by weight, preferably from 0.5 to 20% by weight, particularly preferably from 0.6 to 10% by weight, based in each case on the total weight of the solids of the mixture. By combining suitable IR-absorbing dyes, it is possible to utilize not only narrow IR ranges but the entire wavelength range of the near IR spectrum (700 to 1,200 nm). At least two IR-absorbing dyes may be required for covering the IR range from 700 to 1200 nm, in particular from 800 to 1100 nm.
Any organic, polymeric binder can be used in the present invention. The organic, polymeric binder is preferably a binder having acidic groups with a pKa of less than 13. This pKa helps ensure that the layer is soluble or at least swellable in aqueous alkaline developers. Advantageously, the binder is a polymer or polycondensate, for example a polyester, polyamide, polyurethane or polyurea. Polycondensates and polymers having free phenolic hydroxyl groups, as obtained, for example, by reacting phenol, resorcinol, a cresol, a xylenol or a trimethylphenol with aldehydesxe2x80x94especially formaldehydexe2x80x94or ketones, are also particularly suitable. Condensates of sulfamoyl- or carbamoyl-substituted aromatics and aldehydes or ketones are also suitable. Polymers of bismethylol-substituted ureas, vinyl ethers, vinyl alcohols, vinyl acetals or vinylamides and polymers of phenylacrylates and copolymers of hydroxylphenylmaleimides are likewise suitable. Furthermore, polymers having units of vinylaromatics, N-aryl(meth)acrylamides or aryl (meth)acrylates may be mentioned, it being possible for each of these units also to have one or more carboxyl groups, phenolic hydroxyl groups, sulfamoyl groups or carbamoyl groups. Specific examples include polymers having units of 2-hydroxyphenyl (meth)acrylate, of N-(4-hydroxyphenyl)(meth)acrylamide, of N-(4-sulfamoylphenyl)-(meth)acrylamide, of N-(4-hydroxy-3,5-dimethylbenzyl)(meth)acrylamide, or 4-hydroxystyrene or of hydroxyphenylmaleimide. The polymers may additionally contain units of other monomers which have no acidic units. Such units include vinylaromatics, methyl (meth)acrylate, phenyl(meth)acrylate, benzyl (meth)acrylate, methacrylamide or acrylonitrile. In this context, the term xe2x80x9c(meth)acrylatexe2x80x9d represents acrylate and/or methacrylate. The same applies to xe2x80x9c(meth)acrylamidexe2x80x9d, etc.
Any amount of binder can be used. The amount of the binder is advantageously from 40 to 99.8% by weight, preferably from 70 to 99.4% by weight, particularly preferably from 80 to 99% by weight, based in each case on the total weight of the nonvolatile components of the mixture.
In a preferred embodiment, the polycondensate is a novolak, preferably a cresol/formaldehyde or a cresol/xylenol/formaldehyde novolak, the amount of novolak advantageously being at least 50% by weight, preferably at least 80% by weight, based in each case on the total weight of all binders.
Finally, the properties of the mixture according to the invention can also be influenced or controlled, for example, by including finely divided, non-inhibiting, soluble or dispersible dyes which have virtually no absorption in the IR range. Triarylmethane, azine, oxazine, thiazine and xanthene dyes are particularly suitable for this purpose. The amount of any dyes additionally present in the mixture can be any amount desired advantegeously, from 0.01 to 30% by weight, preferably from 0.05 to 10% by weight, based in each case on the total weight of the nonvolatile components of the mixture.
In addition to the components listed above, the mixture may contain further additives which have no layer-inhibiting activity, e.g. carbon black pigments as additional IR absorbers, surfactants (preferably fluorine-containing surfactants or silicone surfactants), polyalkylene oxides for controlling the acidity of the acidic units and low molecular weight compounds having acidic units for increasing the rate of development (e.g. benzoic acid or para-toluenesulfonic acid) However, the mixture generally contains no components which might influence the daylight sensitivity on exposure to radiation in the ultraviolet or visible range of the spectrum.
Binder and IR-absorbing cyanine dye having a betaine structure or having a betaine structure and containing an anion are generally present as a mixture but may also form separate layers. As a result of the separate arrangement of binder and IR-absorbing dyes, higher photosensitivity and better stability to aqueous alkaline developer solutions can often be achieved. In this embodiment, the dye layer is generally above the binder layer. Owing to the hardness of the dye layer, the sensitivity of the surface of the recording material may be simultaneously reduced. In this embodiment, the dye layer preferably comprises one or more of the cyanine dyes having a betaine structure or having a betaine structure and containing an anion. Most preferably only one is included. Non-IR-sensitive dyes are preferably present only if required, and if included, are generally present in the binder layer underneath.
The present invention furthermore relates to recording materials having a substrate and a positive-working, IR-sensitive layer, wherein the layer comprises a mixture as previously described. However, the mixture according to the invention can also be used for other purposes, e.g. as a photoresist. The invention furthermore relates to recording material having a substrate, a layer which predominantly or completely comprises at least one binder and a layer which comprises or consists essentially of at least one of the described IR-absorbing dyes having a betaine structure or having a betaine structure and containing an anion, or a mixture of these dyes with one or more other dyes such as triarylmethane, azine, oxazine, thiazine and/or xanthene dyes (in the layer sequence). The dye layer may also contain particles having a dulling effect, e.g. SiO2 particles or pigments. Additives for improving the uniformity (such as silicone surfactants or fluorine containing surfactants) may likewise be included in minor amounts.
For the preparation of recording material, any known method can be used. For example, the mixture according to the invention can be dissolved in a solvent mixture which does not react irreversibly with the components of the mixture. The solvent should preferably be tailored to the intended coating method, the layer thickness, the composition of the layer and the drying conditions. Suitable solvents include general ketones, such as methyl ethyl ketone (butanone), as well as chlorinated hydrocarbons, such as trichloroethylene or 1,1,1-trichloroethane, alcohols, such as methanol, ethanol or propanol, ethers, such as tetrahydrofuran, glycol-monoalkyl ethers, such as ethylene glycolmonoalkyl ether or propylene glycolmonoalkyl ether and esters, such as butyl acetate or propylene glycolmonoalkyl ether acetate. It is also possible to use a mixture which, for special purposes, may additionally contain solvents such as acetonitrile, dioxane, dimethylacetamide, dimethylsulfoxide or water. For the preparation of a double layer (binder layer+dye layer), the same or different solvents may be used for the two coating processes.
The substrate in the recording material according to the invention can be any desired, and is preferably an aluminum foil or a laminate comprising an aluminum foil and a polyester film. The aluminum surface is preferably roughened, anodized and hydrophilized with a compound which contains at least one phosphonic acid unit or phosphonate unit as well known in the art. A particularly preferred compound which contains phosphonic acid units is polyvinylphosphonic acid. Before the roughening, degreasing and pickling with alkalis and preliminary mechanical and/or chemical roughening may be effected.
A solution of the mixture according to the invention can then be applied to the substrate and dried. Any suitable thickness of the IR-sensitive layer can be formed and the thickness of the IR-sensitive layer is advantageously from 1.0 to 5.0 xcexcm, preferably from 1.5 to 3.0 xcexcm. In the case of the double layer, the thickness of the binder layer is advantageously from 1.0 to 5.0 xcexcm, preferably from 1.5 to 3.0 xcexcm, while the dye layer is generally substantially thinner in comparison and preferably has a thickness of only from 0.01 to 0.3 xcexcm, more preferably from 0.015 to 0.10 xcexcm.
To protect the surface of the recording material, in particular from mechanical action, an overcoat may also optionally be applied. The overcoat generally comprises at least one water-soluble polymeric binder, such as polyvinyl alcohol, polyvinylpyrrolidone, partially hydrolyzed polyvinyl acetates, gelatin, carbohydrates or hydroxyethylcellulose, and can be produced in any known manner such as from an aqueous solution or dispersion which may, if required, contain small amounts, i.e. less than 5% by weight, based on the total weight of the coating solvents for the overcoat, of organic solvents. The thickness of the overcoat can suitably be any amount, advantageously up to 5.0 xcexcm, preferably from 0.1 to 3.0 xcexcm, particularly preferably from 0.15 to 1.0 xcexcm.
Finally, the present invention also relates to processes for the production of a lithographic printing plate, in which the recording material according to the invention is exposed imagewise to infrared radiation and then developed in a conventional aqueous alkaline developer at a temperature of from 20 to 40xc2x0 C. During development, any water-soluble overcoat present is also removed.
For development, any developers generally customary for positive plates may be used. Silicate-based developers which have a ratio of SiO2 to alkali metal oxide of at least 1 are preferred. This helps to ensure that alumina layer (if present) of the substrate is not damaged. Preferred alkali metal oxides include Na2O and K2O, and mixtures thereof. In addition to alkali metal silicates, the developer may optionally contain further components, such as buffer substances, complexing agents, antifoams, organic solvents in small amounts, corrosion inhibitors, dyes, surfactants and/or hydrotropic agents as well known in the art.
The development is preferably carried out at temperatures of from 20 to 40xc2x0 C. in mechanical processing units as customary in the art. For regeneration, alkali metal silicate solutions having alkali metal contents of from 0.6 to 2.0 mol/l can suitably be used. These solutions may have the same silica/alkali metal oxide ratio as the developer (generally, however, it is lower) and likewise optionally contain further additives. The required amounts of regenerated material must be tailored to the developing apparatuses used, daily plate throughputs, image areas, etc. and are in general from 1 to 50 ml per square meter of recording material. The addition can be regulated, for example, by measuring the conductivity as described in EP-A 0 556 690, which is incorporated herein by reference.
The recording material according to the invention can, if required, then be aftertreated with a suitable correcting agent or preservative as known in the art.
To increase the resistance of the finished printing plate and hence to extend the print run, the layer can be briefly heated to elevated temperatures (xe2x80x9cbakingxe2x80x9d). As a result, the resistance of the printing plate to washout agents, correction agents and UV-curable printing inks also increases. Such a thermal aftertreatment is described, inter alia, in DE-A 14 47 963 and GB-A 1 154 749, which are incorporated herein by reference.
The following examples explain in detail the subject of the invention. In the examples, pbw is part(s) by weight. Percentages and amounts are to be understood in weight units, unless stated otherwise, i.e. percentages are to be understood as percentages by weight unless stated otherwise. Comparative compounds or comparative examples are marked with an asterisk (*).
First, the dissolution-inhibiting or dissolution-imparting properties of the IR dyes are determined by determining the rate of removal of the layer before and after imagewise heating in an aqueous alkaline developer as follows:
1. Preparation of the basic formulation.
2. Addition of the additives to be investigated to the basic formulation.
3. Application of the solutions to a suitable substrate so that, after drying, a layer thickness of 1.9xc2x10.1 xcexcm results.
4. Determination of the rate of removal by development in a cell over a period of 30 seconds to 6 min.
5. If the rate of removal is lower than in the case of a simultaneously measured basic formulation, the additive had a dissolution-increasing property and corresponded to the recording material according to the invention.
6. If the additive had an inhibiting effect, a sample was post-baked at from 50 to 160xc2x0 C. for from 5 to 20 seconds and the rate of removal was determined as described under section 4. A possible loss of layer as a result of the post-bake was taken into account. If the inhibiting effect was maintained in comparison with the basic formulation, this likewise corresponded to the recording material according to the invention.